The goal of this study is the development of materials which will release heparin from their surfaces at a constant rate. This would be accomplished by the incorporation of heparin into biosorbable polymers, such as polylactic acid, polyglycolic acid and thier copolymers, which are presently in use as absorbable suture materials. These polymers do not permit diffusion of molecules with molecular weight over 300. Hence, the time over which the heparin release would occur would be determined by the rate of degradation of the polymer. This time can be varied from days to many months. Among the most important uses for these materials is the fabrication of a composite small-vessel prosthesis, consisting of an inner layer of the heparin-releasing polymer, and an outer layer of a knit or woven permanent prosthesis material. The lumen would be held open by the heparin while stable fibrous tissue was forming around the permanent prosthesis. The fibrous tissue would be gradually exposed to the blood as the inner coating slowly disappeared. There is evidence that such exposue will result in a thin neointima, allowing the prosthesis to be used successfully for small vessels. The heparin-releasing polymer can also be used as a coating for needles, cannulas, syringes, etc., and possibly for membranes (applied as a fine grid). A major advantage of the material is the ability to forestall clotting in blood near the surface by releasing heparin only into the surface layers. Such a small amount is required that the increase in systemic heparin would not be detectable. The same technique can be applied to the release of acetyl-containing molecules from a surface. This raises the possibility of producing a surface which would inactivate platelets in the blood layer adjacent to the surface, inhibiting platelet adhesion and aggregation. Specific materials are proposed for this purpose.